Hybrid vehicles are vehicles with two or more power sources in a drivetrain. The most common automotive hybrid being a gasoline-electric hybrid, having an internal combustion engine and an electric motor/generator. Typically hybrid vehicles use both an engine and a motor to improve fuel consumption, emission, and performance by switching between the two at optimal times when either the engine or the motor may be more efficient than the other, or in a combination of the two when it is more efficient than the use of only one, depending on the hybrid system and vehicle mode.
Hybrid vehicles are generally classified by the division of power between sources; both sources may operate in parallel to simultaneously provide torque to the powertrain or they may operate in series with one source exclusively providing the torque and the second being used to augment the first's power reserve.
Hybrid vehicles may also incorporate an automatic transmission in the driveline between one or more of the sources and the drive wheels. An automatic transmission, also called an automatic gearbox, can automatically shift to different gear ratios as the vehicle starts to move, freeing the driver from having to shift gears manually. Most automatic transmissions have a defined set of gear ranges, and are referred to as a step-ratio automatic transmission.
During a process referred to as regenerative-braking, the motor is turned by the powertrain to charge a high-voltage battery. The motor provides vehicle deceleration by removing torque from the powertrain and transferring it into electric energy for storage in the battery. During regenerative-braking, driver torque demand may be provided solely by regenerative-braking or supplemented by frictional-braking. The apportionment of driver torque demand between frictional-braking torque and regenerative-braking torque (i.e., negative input torque via the motor) is balanced through the braking process to achieve as much regeneration as possible so that the energy may later be used to propel the vehicle and thereby improve fuel economy.
During regenerative-braking, it may be advantageous to downshift the transmission to increase the speed of the motor allowing for the motor to provide a greater negative torque input into the powertrain and provide for an increase in battery charging. However, the torque transfer through the transmission during a regenerative-braking downshift may need to be controlled to reduce shift shock. An example of a control strategy to reduce shift shock during a regenerative-braking downshift may be found in United States Patent Application US 2013/0296100, which is hereby incorporated by reference.